# LeetCode 230. Kth Smallest Element in a BST

## Description

https://leetcode.com/problems/kth-smallest-element-in-a-bst/

Given the `root` of a binary search tree, and an integer `k`, return the `kth` (1-indexedsmallest element in the tree.

Example 1:

```Input: root = [3,1,4,null,2], k = 1
Output: 1
```

Example 2:

```Input: root = [5,3,6,2,4,null,null,1], k = 3
Output: 3
```

Constraints:

• The number of nodes in the tree is `n`.
• `1 <= k <= n <= 104`
• `0 <= Node.val <= 104`

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?

## Explanation

In-order traverse to get node values in ascending order. Then get the k – 1 th value.

## Python Solution

``````# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
def kthSmallest(self, root: TreeNode, k: int) -> int:
values = []

self.inorder_traverse(root, values)

return values[k - 1]

def inorder_traverse(self, root, results):
if not root:
return

self.inorder_traverse(root.left, results)
results.append(root.val)
self.inorder_traverse(root.right, results)``````
• Time complexity: O(N).
• Space complexity: O(N).